//Wed 17 Feb 2010 07:52:21 PM CST 
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>

using namespace std;

class Archery
{
public:
	double expectedPoints(int N, vector <int> ringPoints)
	{
//		double area = 1.0;
		double sum = 0.0;
		for(int i=0; i<=N; i++)
		{
			sum += 1.0 * ringPoints[i] * (2.0*i+1);
		}
		return sum / pow(1.0*(N+1), 2.0);
	}
};
